FIG. 8A and FIG. 8B show a configuration of a conventional audio playback system mounted in a vehicle. Further, FIG. 8A is a cross-sectional view showing the vehicle from the side and FIG. 8B is a top view showing the vehicle from above.
In FIG. 8A and FIG. 8B, the conventional audio playback system is equipped in a passenger interior 2 of a sedan vehicle 1 and is provided with a subwoofer 5 in the rear shelf in the passenger interior 2. The Subwoofer 5 is mounted on a trunk 6 as an enclosure, while being directed toward the passenger interior 2.
In case of the sedan vehicle 1, to reproduce bass sound in the passenger interior 2, the subwoofer 5 having about a 20-cm caliber is provided in the rear shelf of the passenger interior and an audio signal of a frequency band of 120 Hz or lower is reproduced therefrom.
If other parts mounted in the vehicle make it difficult to secure sufficient space for installing a subwoofer in the rear shelf, an audio signal having a frequency band of 150 Hz or lower is reproduced by installing a subwoofer of a 16-cm caliber together with an enclosure of, for example, 7 liters to 15 liters, under the driver's seat.
Further, in case of a wagon vehicle, which is popular recently, given that no rear shelf is provided, a subwoofer is generally installed under the driver's seat as described above.
Alternatively, taking into account the resonance modes of the passenger interior, there is a technique of arranging a subwoofer at a location over which the node of the first-order resonance mode and the node of the third-order resonance mode cross.
Furthermore, Patent Document 1 discloses a technique of installing a subwoofer near the joint part of the rear door over which the node of the first-order resonance mode and the node of the third-order resonance mode of the sedan vehicle cross and reproducing the frequency band of the third-order resonance frequency or lower from the subwoofer.
The above resonance mode and resonance frequency will be described.
“Resonance” in the present description refers to a phenomenon of resonance of sound determined by the shape and dimensions of the passenger interior, as in a Helmholtz resonator. A resonance frequency refers to a specific discrete frequency causing the phenomenon of resonance. A resonance mode refers to distribution of sound pressure in case that sound is reproduced in the passenger interior at a resonance frequency, and, in the passenger interior, an area where sound pressure increases significantly is referred to as an “antinode” and an area where sound pressure decreases is referred to as a “node.” If a speaker is installed in the antinode of a specific resonance mode and sound is reproduced in the passenger interior at a resonance frequency matching this resonance mode, the phenomenon of resonance occurs and the area where sound pressure is high and the area where sound pressure is low appear remarkably. According to the theory, although there are an infinite number of resonance frequencies and resonance modes in the passenger interior, what is particularly significant upon design of the audio playback system is some modes in a lower frequency band (hereinafter “low band”).
The resonance frequencies and resonance modes can be determined according to numerical analysis methods such as the finite element method. For example, in case of a sedan vehicle of 2000 cc displacement, the first-order resonance frequency is about 80 Hz, and the frontmost end and rearmost end of the vehicle provide the antinodes in the first-order resonance mode and the plane which is vertical to the traveling direction near the center of the passenger interior provides the nodes of the first-order resonance mode. Further, the second-order resonance frequency is about 130 Hz, an area above the dashboard in the passenger interior and an area around the footwell of the front seats provide the antinodes in the second-order resonance mode, and the plane which is virtually parallel to the ground at the height near the shoulders of the passengers, provides the nodes of the second-order resonance mode. In addition to the above numerical analysis method, nodes and antinodes can be measured and determined by installing a speaker and a microphone in an actual vehicle.
FIG. 9A to C show the nodes and antinodes in the first- to third-order resonance modes of the vehicle. Further, FIG. 9A is a perspective view of the vehicle, FIG. 9B is a cross-sectional view showing the vehicle from the side and FIG. 9C is a plan view showing the vehicle from above.
As shown with dotted lines in FIG. 9A to FIG. 9C, nodes a to c in the first- to third-order resonance modes appear and antinodes d and e of the resonance modes shown by hatching in FIG. 9B and FIG. 9C appear in the passenger interior.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI7-131884